The present invention relates to a gas-barrier film which is produced through application of a metallic compound to the surface of a polymer layer formed from a specific polymer, and more particularly to a gas-barrier film which is produced through application of a metallic compound to a polymer layer formed from a mixture of a poly(meth)acrylic acid polymer and a polyalcohol such as sugar. The present invention provides a gas-barrier film which exhibits excellent oxygen-gas-barrier properties, particularly in an atmosphere of high humidity, and which is suitably used in sterilization treatment such as retorting or boiling.
Conventionally, in order to enhance gas-barrier properties of plastic films, several processes have been proposed. For example, Japanese Patent Application Laid-Open (kokai) No. 9-157406 discloses a process in which an inorganic layer compound serving as a filler is incorporated into a plastic film, and Japanese Patent Application LaidOpen (kokai) No. 4-366142 discloses a process in which an inorganic compound is deposited on the surface of a plastic film. In the former process, in order to enhance gas-barrier properties of a plastic film, a large amount of inorganic layer compound must be incorporated into the film, and thus properties of a matrix resin, such as transparency and mechanical strength, are impaired. In the latter process, deposition is carried out at high temperature to form a thin film, and a plastic layer may soften due to heat load, and thus a heat-resistant plastic such as polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, or polyimide must be exclusively employed. Also, the latter process is disadvantageous in that when a resin of low Young""s modulus is employed, gas-barrier properties of a produced film may deteriorate, since the tensile strength of the resin decreases during deposition and the deposited film is prone to crack.
The latter process is also disadvantageous in that operation is cumbersome and an expensive apparatus is required, since deposition must be carried out in a vacuum apparatus. Japanese Patent Application Laid-Open (kokai) No. 8-142256 discloses a moisture-resistant composite deposition film comprising at least one laminated structure containing a polymer film substrate (A), a deposition film (B) formed of inorganic material which is laminated on at least one surface of the substrate (A), a water-resistant film (C) formed of a sugar and a polycarboxylic acid or a partially neutralized polycarboxylic acid, the film (C) being laminated on the film (B); and a polymer composition layer (D) containing a drying agent, such that the layer (D) is provided on at least one side of the laminated structure. However, the composition film may involve drawbacks similar to those as described above, since a deposition film is employed in the composite film. Therefore, there has been demand for a process for producing a gas-barrier film more simply and conveniently.
In view of the foregoing, an object of the present invention is to provide a gas-barrier film which can be produced through a simple, convenient process and which exhibits excellent gas-barrier properties.
The present inventors have found that the aforementioned problems can be solved by a gas-barrier film which is produced through application of a layer containing a metallic compound (hereinafter the layer may be referred to as a xe2x80x9cmetallic-compound-containing layerxe2x80x9d) to the surface of a polymer layer formed from a mixture of a polyalcohol and at least one poly(meth)acrylic acid polymer selected from the group consisting of poly(meth)acrylic acids and partially neutralized poly(meth)acrylic acids. The present invention has been accomplished on the basis of this finding. Japanese Patent Application Laid-Open (kokai) No. 8-224825 discloses a gas-barrier laminate which is produced from a plastic film and a metallic compound. Japanese Patent Application LaidOpen (kokai) No. 58-128852 discloses a laminate exhibiting excellent adhesion, which is produced from a plastic film and a carboxyl-group-containing polyolefin film with the intervention of a metallic compound. In the aforementioned laminates, metallic compound layers of continuous phase are formed through deposition or sputtering.
Accordingly, in a first aspect of the present invention, there is provided a gas-barrier film which is produced through applying a layer containing a metallic compound to the surface of a polymer layer formed from a mixture of a polyalcohol and at least one poly(meth)acrylic acid polymer selected from the group consisting of poly(meth)acrylic acids and partially neutralized poly(meth)acrylic acids. There is also provided a gas-barrier film wherein the surface of the polymer layer to which the metallic-compound-containing layer is not applied is fixed onto a surface of a substrate. There is also provided a gas-barrier film wherein at least the polymer layer is subjected to heat treatment. There is also provided a gas-barrier film wherein the metallic compound is at least one species selected from the group consisting of magnesium oxide, calcium oxide, zinc oxide, magnesium hydroxide, calcium hydroxide, and zinc hydroxide. There is also provided a gas-barrier film wherein the metallic-compound-containing layer is produced from a mixture of the metallic compound and a resin. There is also provided a gas-barrier film for use in sterilization treatment.
In a second aspect of the present invention, there is provided a laminated gas-barrier film comprising a gas-barrier film as recited in the first aspect, wherein a plastic film is laminated on either surface of the gas-barrier film.
The present invention will next be described in detail.
In the present invention, a metallic-compound-containing layer which is applied to the surface of a polymer layer may be a layer containing a metallic compound alone, or a layer containing a resin in which a metallic compound is mixed or dispersed (hereinafter the layer will be referred to as xe2x80x9clayer of a mixture of metallic compound and resinxe2x80x9d). As used herein, the phrase xe2x80x9ca metallic-compound-containing layer is applied to the surface of a polymer layerxe2x80x9d refers to xe2x80x9ca metallic compound or a suspension of a mixture of metallic compound and resin is applied or sprayed to the surface of a polymer layer,xe2x80x9d xe2x80x9cthe metallic compound or the suspension is applied to the surface of a polymer layer through dipping,xe2x80x9d or xe2x80x9cpowder of the metallic compound is applied to the surface of a polymer layer through powdering or spraying.xe2x80x9d The phrase does not refer to application of a metallic-compound-containing layer through deposition or sputtering.
The metallic-compound-containing layer of the gas-barrier film of the present invention differs in terms of surface roughness from a metallic compound layer of continuous phase which is produced through deposition or sputtering. The metallic-compound-containing layer may have a non-continuous phase or a continuous phase. In the present invention, a metallic-compound-containing layer can be introduced through a convenient process as described below, such as powdering of a metallic compound, or application or spraying of a suspension of the metallic compound. As described above, a metallic-compound-containing layer which contains a heat-resistant resin of high Young""s modulus as a plastic film and which is produced through deposition or sputtering, which processes require complicated operation and expensive apparatus, has a mean surface roughness (Ra) of 0.0002-0.002 xcexcm as measured by use of an atomic force microscope (AFM), or an Ra of 0.0001-0.002 xcexcm as calculated through the below-described method making use of a micrograph of transmission electron microscope (TEM). In contrast, in the present invention, a layer containing a metallic compound alone or a layer of a mixture of metallic compound and resin has an Ra of 0.003-0.03 xcexcm as measured by AFM, preferably 0.003-0.02 xcexcm, and an Ra of 0.003-5 xcexcm as calculated by use of a TEM micrograph, more preferably 0.01-3 xcexcm.
In the film of the present invention in which a metallic-compound-containing layer is applied to the surface of a polymer layer formed from a mixture of a poly(meth)acrylic acid polymer and a polyalcohol, a metal invades the polymer layer from the metallic-compound-containing layer. As described below, invasion of a metal can be confirmed by means of energy-dispersive X-ray spectroscopy (EDX). The existence ratio in the polymer layer (the number of counting of metallic atoms/the number of counting of oxygen atoms) is 0.1-20 at a position 0.1 xcexcm deep in a polymer layer from the interface between the polymer layer and a layer containing a metallic compound solely or a layer of a mixture of metallic compound and resin, preferably 0.5-10. When the existence ratio is high, the amount of a metallic compound in a polymer layer is large.
The present invention provides a gas-barrier film which is produced through application of a metallic-compound-containing layer to the surface of a polymer layer, the polymer layer being produced through drying of a mixture of a polyalcohol and at least one poly(meth)acrylic acid polymer selected from the group consisting of poly(meth)acrylic acids and partially neutralized poly(meth)acrylic acids. The present invention also provides a gas-barrier film in which the surface of the polymer layer to which the metallic-compound-containing layer is not applied is fixed onto a substrate layer. In the present invention, in order to impart some degree of water-resistance and gas-barrier properties to the polymer layer, at least the polymer layer is preferably subjected to heat treatment. In addition, the polymer layer must be adjacent to a metallic-compound-containing layer or a layer of a mixture of metallic compound and resin.
[Poly(meth)acrylic acid polymer]
As used herein, the term xe2x80x9cpoly(meth)acrylic acid polymerxe2x80x9d refers to acrylic acid polymers or methacrylic acid polymers containing two or more carboxyl groups, and to carboxylic acid polymers or partially neutralized carboxylic polymers. Specific examples of poly(meth)acrylic acids include a polyacrylic acid, a polymethacrylic acid, a copolymer of acrylic acid and methacrylic acid, and a mixture of two or more species thereof. In the present invention, a copolymer of acrylic acid or methacrylic acid, and a methyl ester or ethyl ester thereof may be employed, so long as the copolymer can be dissolved in a solvent such as water or alcohol, or in a solvent mixture of water and alcohol. Of the aforementioned examples, a homopolymer of acrylic acid or methacrylic acid, or a copolymer of acrylic acid and methacrylic acid is preferable. In consideration of oxygen-gas-barrier properties, a homopolymer of acrylic acid or a copolymer of acrylic acid and methacrylic acid in which the amount of acrylic acid is greater than that of methacrylic acid is more preferable. The number average molecular weight of a poly(meth)acrylic acid polymer is not particularly limited, but in consideration of handling, the number average molecular weight is preferably 1,000-4,000,000, more preferably 2,000-250,000.
Partially neutralized poly(meth)acrylic acid may be produced by partially neutralizing carboxyl groups of poly(meth)acrylic acid with an alkali (i.e., by producing carboxylic acid salts). Examples of alkalis which may be employed include alkali metal hydroxides such as sodium hydroxide, lithium hydroxide, and potassium hydroxide; and ammonium hydroxide. Partially neutralized poly(meth)acrylic acid is typically produced by adding an alkali to an aqueous solution of poly(meth)acrylic acid for reaction between the alkali and the acid. Partially neutralized poly(meth)acrylic acid may be an alkali metal salt or ammonium salt of the acid. Such an alkali metal salt or ammonium salt is contained in a polymer layer as a monovalent metal or an ammonium ion. When partially neutralized poly(meth)acrylic acid is employed, coloring of a polymer layer due to heat may be suppressed. Therefore, partially neutralized poly(meth)acrylic acid is preferably employed in accordance with needs.
The degree of neutralization of partially neutralized poly(meth)acrylic acid may be desirably determined by regulating the ratio of the amount of poly(meth)acrylic acid to that of an alkali. The degree of neutralization of partially neutralized poly(meth)acrylic acid is preferably determined on the basis of oxygen-gas-barrier properties of a produced film. The degree of neutralization may be obtained by use of the following formula: degree of neutralization (%)=(N/N0)xc3x97100. In the formula, N represents the amount by mol of neutralized carboxyl groups in 1 g of partially neutralized poly(meth)acrylic acid; and No represents the amount by mol of carboxyl groups in 1 g of non-neutralized poly(meth)acrylic acid.
According to Japanese Patent Application Laid-Open (kokai) No. 7-165942, oxygen-gas-barrier properties of a film produced from at least one poly(meth)acrylic acid polymer (A) selected from the group consisting of poly(meth)acrylic acids and partially neutralized poly(meth)acrylic acids and a polyalcohol polymer (B)xe2x80x94the film being employed in a polymer layer of the gas-barrier film of the present inventionxe2x80x94are affected by heat treatment conditions during formation of the film, or by the degree of neutralization of the polymer (A) when the mixture ratio of these two polymers is constant. As compared with the case in which poly(meth)acrylic acid is employed as the polymer (A), when neutralized poly(meth)acrylic acid is employed, oxygen-gas-barrier properties of a produced film tend to be enhanced. However, when the degree of neutralization increases, oxygen-gas-barrier characteristic of the produced film tend to deteriorate once it has reached a maximal value (i.e., a minimal value of oxygen permeability). When the degree of neutralization is in excess of 20%, oxygen-gas-barrier properties of a produced film deteriorate as compared with the case in which non-neutralized poly(meth)acrylic acid is employed.
Therefore, from the viewpoint of oxygen-gas-barrier properties, non-neutralized poly(meth)acrylic acid or partially neutralized poly(meth)acrylic acid (the degree of neutralization: 20% or less) is preferably employed as poly(meth)acrylic acid polymer constituting a polymer layer in the gas-barrier film of the present invention. More preferably, non-neutralized poly(meth)acrylic acid or partially neutralized poly(meth)acrylic acid (degree of neutralization: 15% or less) is employed. Much more preferably, partially neutralized poly(meth)acrylic acid (degree of neutralization: 1-13%) is employed.
[Polyalcohol]
As used herein, the term xe2x80x9cpolyalcoholxe2x80x9d refers to low molecular weight compounds containing two or more hydroxyl groups, alcohol polymers, polyvinyl alcohols (PVA), sugars, and starches. Examples of low molecular weight compounds containing two or more hydroxyl groups include glycerin, ethylene glycol, propylene glycol, 1,3-propanediol, pentaerythritol, polyethylene glycol, and polypropylene glycol. PVA which may be employed has a saponification percentage of 95% or more, preferably 98% or more, and has an average degree of polymerization of 300-1,500. In consideration of compatibility with poly(meth)acrylic acid polymer, a vinyl alcohol-poly(meth)acrylic acid copolymer predominantly containing vinyl alcohol may be employed. Sugars which may be employed include monosaccharides, oligosaccharides, and polysaccharides. Such sugars also include sugar alcohols such as sorbitol, mannitol, dulcitol, xylitol, erythritol, and a variety of substitution compounds and derivatives thereof, which are disclosed in Japanese Patent Application Laid-Open (kokai) No. 7-165942. Such sugars preferably have solubility in water, alcohol, or a solvent mixture of water and alcohol.
Starches are a class of polysaccharides. Example of starches which may be employed in the present invention include plant starches (unmodified starches) such as wheat starch, corn starch, glutinous corn starch, potato starch, tapioca starch, rice starch, sweet potato starch, and sago starch; and a variety of modified starches. Examples of modified starches include physically modified starches, enzymatically modified starches, chemical-decomposition modified starches, chemically modified starches, and graft starches which are produced through graft-polymerization of starches and a monomer. Of these starches, water-soluble modified starch which is produced by, for example, hydrolyzing potato starch with an acid is preferable. Sugar alcohol which is produced by substituting an end group (aldehyde group) of starch with a hydroxyl group is more preferable. Starches may be in the form of a hydrate. These starches may be employed singly or in combination of two or more species.
In consideration of production of a polymer layer exhibiting excellent oxygen-gas-barrier properties under high humidity, the mixture ratio (by weight) of poly(meth)acrylic acid polymer to polyalcohol is preferably 99:1xe2x88x9220:80, more preferably 95:5xe2x88x9240:60, much more preferably 95:5xe2x88x9250:50.
Preparation and formation of the polymer layer which is a constituent of the present invention will next be described. A mixture of a poly(meth)acrylic acid polymer and a polyalcohol is prepared through any of the following methods: a method in which each component is dissolved in water; a method in which aqueous solutions of the components are mixed with each other; and a method in which an acrylic acid monomer is polymerized in an aqueous solution of a polyalcohol. When an acrylic acid monomer is polymerized in an aqueous solution of a polyalcohol, if desired, the resultant poly(meth)acrylic acid is neutralized with an alkali. When poly(meth)acrylic acid and, for example, sugar are dissolved in water, an aqueous solution of uniform mixture is obtained. Instead of water, a solvent such as alcohol, or a solvent mixture of water and alcohol may be employed.
When a polymer layer is subjected to heat treatment for imparting water resistance and further enhanced gas-barrier properties to the product, in order to moderate the conditions of the treatment, a water-soluble metallic salt of inorganic or organic acid may appropriately be added to an aqueous solution of a mixture of the above polymers during preparation of the solution. A metal of the salt may be an alkali metal such as lithium, sodium, or potassium. Specific examples of metallic salts of inorganic or organic acid include lithium chloride, sodium chloride, potassium chloride, sodium bromide, sodium phosphite (sodium hypophosphite), disodium hydrogenphosphite, disodium phosphate, sodium ascorbate, sodium acetate, sodium benzoate, and sodium hyposulfite. A phosphine acid metallic salt (hypophosphorous acid metallic salt) which is at least one species selected from among phosphine acid metallic salts (hypophosphorous acid metallic salts) such as sodium phosphite (sodium hypophosphite) and calcium phosphite (calcium hypophosphite) is preferable. The amount of a metallic salt of inorganic or organic acid which is added is preferably 0.1-40 parts by weight on the basis of the solid content of a solution of a mixture of the polymers, more preferably 1-30 parts by weight.
The process for forming a polymer layer from the above-prepared composition is not particularly limited. For example, a polymer layer is obtained through any of the following processes: a solution-cast process in which an aqueous solution of a polymer mixture is applied onto a support (substrate) and dried to form a film; an extrusion process in which an aqueous solution containing a polymer mixture at high concentration is cast through a tiny space by use of an extruder under ejection pressure, and the resultant water-containing film is dried on a rotary drum or belt; and a process in which an aqueous solution containing a polymer mixture at high concentration is applied onto a plastic film, and the film is stretched under heating. Alternatively, when a substrate has a complicated shape, the substrate may be dipped in a solution of a raw material composition, to thereby coat the surface of the substrate with the resultant film. A dried film which is produced as described above will be referred to as xe2x80x9ca polymer layer.xe2x80x9d of the aforementioned processes, a solution-cast process (cast process or coating process) is preferably employed, since a polymer layer (dried film) of excellent transparency can be easily produced.
When a polymer layer is produced through a solution-cast process, the solid content of a solution of the polymer mixture is preferably 1-30 wt. %. When an aqueous solution of a polymer mixture is prepared, if necessary, a solvent other than water, such as alcohol, or a softening agent may appropriately be added to the solution. Alternatively, a plasticizer (excluding a low molecular weight compound containing two or more hydroxyl groups), a heat stabilizer, or an inorganic lamella-structured compound such as smectic ore may be incorporated in advance into at least one of poly(meth)acrylic acid polymer and polyalcohol. The thickness of the polymer layer is appropriately determined in accordance with the purpose of use of a final product. The thickness is not particularly limited, but is preferably 0.01-100 xcexcm, more preferably 0.1-50 xcexcm.
In a coating process, for example, a solution of a mixture of poly(meth)acrylic acid and sugar is applied onto a support (substrate) such as a metallic plate, a glass plate, or a plastic plate so as to attain a desired thickness, by use of an apparatus such as an air knife coater, a kiss roll coater, a metaling bar coater, a gravure roll coater, a reverse roll coater, a dip coater, or a die coater, or by use of a combination thereof. Subsequently, the thus-applied solution is dried by evaporating water through spraying of hot air or radiation of IR rays by use of an apparatus such as an arch drier, a straight bath drier, a tower drier, a floating drier, or a drum drier, or by use of a combination thereof, to thereby form a film (processed-polymer layer).
Subsequently, a layer constituted solely by a metallic compound or a layer of a mixture of metallic compound and resin, serving as a metallic-compound-containing layer, is applied to the surface of a polymer layer which is fixed onto a substrate. Preferred examples of metals constituting a metallic compound include alkali metals such as lithium, sodium, calcium, rubidium, and cesium; alkaline earth metals such as beryllium, magnesium, calcium, strontium, and barium; and transition metals having an oxidation number of +2 such as zinc. Examples of metallic compounds which may be employed include metallic elements; inorganic salts such as oxides, hydroxides, halides, and carbonates; organic salts such as carboxylates and sulfonates; and polyacid salts such as poly(meth)acrylates. Of these, oxides, hydroxides, or carbonates of alkaline earth metal or transition metal having an oxidation number of +2 are preferable. More preferably, in consideration of handling and adhesion to a polymer layer, there is employed at least one metallic compound which is selected from among magnesium oxide, calcium oxide, magnesium hydroxide, calcium hydroxide, zinc oxide, zinc hydroxide, magnesium carbonate, and calcium carbonate. A metallic compound preferably has the form of particles.
The method for application of a metallic-compound-containing layer is not particularly limited, so long as the layer is adjacent to a polymer layer. In the case of application of a layer containing a metallic compound alone, the layer is applied to a polymer layer through, for example, any of the following processes: a process in which a metallic compound is powdered; a process in which a metallic compound is dispersed in a solvent and then the resultant suspension is applied onto a polymer layer by use of a gravure roll coater, a reverse roll coater, a dip coater, or a die coater; and a process in which such a suspension is sprayed to a polymer layer. In the present invention, the thus-applied metallic compound particle layer may have a continuous phase or non-continuous phase.
The solvent for preparing a suspension of metallic compound is not particularly limited, and water, any of a variety of solvents, or solvent mixtures may be employed. In consideration of dispersibility of metallic compound particles, applicability of a suspension of metallic compound, or handling of the suspension, a solvent is arbitrarily chosen from among alcohols, aliphatic hydrocarbons, and aromatic compounds. The solvent is preferably an alcohol having a carbon number of 10 or less. When a layer constituted solely by a metallic compound is employed, the layer is not necessarily applied to the entire surface of a polymer layer as a deposition film is required to do so. The amount of the metallic compound that is applied to the polymer layer is preferably 0.01-20 g/m2, more preferably 0.03-10 g/m2, much more preferably 0.06-5 g/m2. When the amount is in excess of the upper limit of the above range, application cannot be carried out, due to splashing of a metallic compound, whereas when the amount is below the lower limit of the above range, a produced film exhibits insufficient gas-barrier properties.
In the case of application of a layer of a mixture of metallic compound and resin, the layer contains at least one resin which is selected from among an alkyd resin, a melamine resin, an acrylic resin, a urethane resin, nitrocellulose, an epoxy resin, a polyester resin, a phenol resin, an amino resin, a fluorine-containing resin, and isocyanate. The weight ratio of a metallic compound to a resin (i.e., metallic compound/resin) is preferably 0.01-1,000, more preferably 0.01-100. A mixture of a metallic compound and a resin may be dissolved or dispersed in an organic solvent to prepare a dispersion or a suspension, after which the resultant dispersion or suspension is applied or sprayed to a polymer layer. The case in which a mixture of a metallic compound and a resin is employed is more preferable than the case in which a metallic compound is employed alone, since a metallic compound is uniformly applied to a polymer layer when the mixture is employed. In order to prepare a dispersion or a suspension, there may be employed any of the solvents which may be employed in the case in which a metallic compound is solely applied. When a mixture of metallic compound and resin is applied, the amount of the mixture which is applied to a polymer layer may be determined by regulating the amount of a metallic compound being preferably 0.03-20 g/m2, more preferably 0.06-10 g/m2, much more preferably 0.06-5 g/m2.
When a metallic compound is applied through deposition or sputtering, the resultant metallic-compound-applied surface may have a mean surface roughness (Ra) of less than 0.003 xcexcm as measured by AFM or as calculated by use of a TEM micrograph. However, such an application process is disadvantageous, in that a resin employed must have heat resistance and high Young""s modulus, and that operation is cumbersome and an expensive apparatus is required, since deposition or sputtering must be carried out in a vacuum apparatus. When the Ra of a metallic-compound-containing layer is in excess of 0.03 xcexcm as measured by AFM or is in excess of 5 xcexcm as calculated by use of a TEM micrograph, adhesion between the metallic-compound-containing layer and a polymer layer is poor, which is not practical. Incidentally, metallic atoms are present in a polymer layer of the gas-carrier film of the present invention in which a metallic-compound-containing layer is applied to the polymer layer produced from a mixture of a poly(meth)acrylic acid polymer and a polyalcohol; or metallic atoms are present in a polymer layer of a laminated gas-barrier film comprising the gas-barrier film and a plastic film laminated thereon. As described below, invasion of a metal into a polymer layer can be confirmed by means of EDX, and the existence ratio in the polymer layer (number of counting of metallic atoms/number of counting of oxygen atoms) is 0.1-20, preferably 0.5-10, at a position 0.1 xcexcm deep in a polymer layer from the interface between the polymer layer and a layer constituted by a metallic compound alone or a layer of a mixture of metallic compound and resin. When the degree is less than 0.1, the polymer layer exhibits insufficient gas-barrier properties, whereas the ratio is in excess of 20, a polymer layer may break, and thus exhibits insufficient gas-barrier properties.
In order to enhance water resistance and gas-barrier properties of a polymer layer which is fixed onto a substrate, at least the polymer layer may be subjected to heat treatment. A polymer layer to which a metallic compound is applied may be subjected to heat treatment under specific conditions, or a metallic compound may be applied to the surface of a polymer layer after the polymer layer is subjected to heat treatment.
As used herein, the term xe2x80x9ca polymer layer which is fixed onto a substratexe2x80x9d refers to xe2x80x9ca polymer layer, to which a metallic-compound-containing layer is not applied, which is fixed onto a substratexe2x80x9d or xe2x80x9ca polymer layer which may be peeled off a substrate.xe2x80x9d The material of a substrate is not particularly limited, and a metallic plate, a glass plate, or a plastic film may be employed as a substrate. Of these, a plastic film is preferably employed. More preferably, a substrate is chosen from a variety of plastic films in accordance with heat treatment temperature or the intended use of a gas-barrier film (for example, a gas-barrier film is used in sterilization treatment). Examples of materials of plastic film include polyester resins such as polyethylene terephthalate (PET), polybutylene terephthalate (PBT), and polyethylene naphthalate (PEN); polyamides such as nylon 6, nylon 66, nylon 12, a nylon 6xe2x80x266 copolymer, a nylon 6xe2x80x212 copolymer, a methaxylylenadipamide nylon 6 copolymer, and amorphous nylon; polyolefins such as low density polyethylene, high density polyethylene, linear low density polyethylene, an ethylene.vinyl acetate copolymer, polypropylene, an ethylene.acrylic acid copolymer, an ethylene.acrylate copolymer, an ethylene.ethyl acrylate copolymer, and poly(methylpentene); polyvinylidene chloride; and polyphenylene sulfide. In order to enhance adhesion between a polymer layer and a substrate layer, an anchoring agent may be applied to the substrate layer.
A polymer layer is subjected to heat treatment under the conditions disclosed in Japanese Patent Application Laid-Open (kokai) No. 8-41218. Specifically, when a sugar is employed as a polyalcohol, a polymer layer is preferably subjected to heat treatment under conditions satisfying the relations between heat treatment temperature and heat treatment time as specified by the following formulas (a) and (b):
(a) log txe2x89xa7.0.0253xc3x97T+11.2
(b) 373xe2x89xa6Txe2x89xa6573
[wherein t represents heat treatment time (minutes), and T represents heat treatment temperature (K)].
When a polymer layer is subjected to heat treatment under the above conditions, the resultant layer has water resistance. In addition, there can be produced a polymer film exhibiting excellent gas-barrier properties; i.e., a polymer layer having a thickness of 2 xcexcm has an oxygen permeability of 2.0xc3x9710xe2x88x9212 mol/m2xc2x7sxc2x7Pa (400 cm3/m2xc2x724hxc2x7atm) or less as measured at 30xc2x0 C. and 80% RH.
A polymer layer, such as a polymer film, a laminate comprising a substrate and a film, or a polymer layer to which a metallic-compound-containing layer is applied, may be subjected to heat treatment by placing the layer in an oven, which is then heated at a predetermined temperature for a predetermined time. Alternatively, a polymer layer may be continuously subjected to heat treatment by passing the layer through an oven which is heated at a predetermined temperature within a predetermined time, or by bringing the layer into contact with a heat roll. After completion of heat treatment, the resultant polymer layer has water resistance and exhibits excellent gas-barrier properties under high humidity. In addition, the resultant polymer layer is insoluble in water or boiling water, and has water resistance as specified as follows. As used herein, the phrase xe2x80x9ca polymer layer has water resistancexe2x80x9d refers to the case in which, when a film comprising a polymer layer is dipped in boiling water for 30 minutes and then dried, the thickness of the polymer layer is 50% or more that of the polymer layer before being dipped in boiling water.
In order to impart strength or sealability to the gas-barrier film of the present invention, a plastic film may further be laminated on the gas-barrier film, to thereby form a laminated gas-barrier film. The type of a laminated gas-barrier film is not particularly limited. Specific examples of layer structures of laminated gas-barrier film include paper/polyethylene terephthalate layer/polymer layer/metallic compound layer/non-stretched polypropylene layer; polyethylene terephthalate layer/polymer layer/metallic compound layer/non-stretched polypropylene layer; polyethylene terephthalate layer/polymer layer/metallic compound layer/linear low density polyethylene layer; polyethylene terephthalate layer/polymer layer/metallic compound layer/low density polyethylene layer; polyethylene terephthalate layer/polymer layer/metallic compound layer/ethylenic copolymer produced by use of a metallocene catalyst; polyethylene terephthalate layer/polymer layer/metallic compound layer/propylenic copolymer produced by use of a metallocene catalyst; stretched nylon layer/polymer layer/metallic compound layer/non-stretched polypropylene layer; stretched nylon layer/polymer layer/metallic compound layer/linear low density polyethylene layer; stretched nylon layer/polymer layer/metallic compound layer/low density polyethylene layer; stretched nylon layer/polymer layer/metallic compound layer/ethylenic copolymer produced by use of a metallocene catalyst; and stretched nylon layer/polymer layer/metallic compound layer/propylenic copolymer produced by use of a metallocene catalyst. The aforementioned metallic compound layer may be a layer containing a metallic compound alone or a layer of a mixture of metallic compound and resin.
In order to produce the aforementioned laminated film, a plastic film layer formed of a thermoplastic resin may be laminated on either or both of the surface of a substrate and the surface of a metallic-compound-containing layer, with or without intervention of an adhesive layer, by means of a known lamination method such as coating, dry lamination, or extrusion coating. In dry lamination, a plastic film or sheet formed of a thermoplastic resin is laminated on a metallic-compound-containing layer or on a second surface of a substrate of a gas-barrier film, the gas-barrier film comprising a polymer layer which is fixed onto a first surface of the substrate, and a metallic-compound-containing layer which is applied onto the polymer layer. In extrusion coating, a thermoplastic resin is melt-extruded on a substrate layer or on a metallic compound layer which is applied onto a polymer layer, and a plastic film is laminated thereon, to thereby produce a laminated film.
One of the outer layers of a laminated gas-barrier film is preferably produced from a material which enables heat sealing, high-frequency sealing, or ultrasonic sealing, (i.e., sealant), in consideration of heat adhesion of the films when a bag is produced from the films. Examples of resins which enable heat sealing include polyolefins such as low density polyethylene, linear low density polyethylene, high density polyethylene, an ethylene.vinyl acetate copolymer, an ethylenic copolymer produced by use of a metallocene catalyst, a propylenic copolymer produced by use of a metallocene catalyst, non-stretched polypropylene, an ethylene.acrylic acid copolymer, an ethylene.acrylic acid salt copolymer, and an ethylene.ethyl acrylate copolymer; and nylon copolymers such as a nylon 6xe2x80x266 copolymer and a nylon 6xe2x80x212 copolymer. Examples of resins which enable high-frequency sealing include polyvinyl chloride, polyvinylidene chloride, nylon 6, and nylon 66. Examples of types of sealing include four-corner sealing, three-corner sealing, butt sealing, and envelope sealing.
The gas-barrier film of the present invention and the laminated gas-barrier film produced therefrom exhibit excellent oxygen-gas-barrier properties in an atmosphere of high humidity. Therefore, the films are suitably employed for packaging material of beverages or foods susceptible to oxygen, such as furikake (processed seasoning granules), wine, dried bonito, miso, ketchup, and snacks. Particularly, the films are suitably employed for packaging material of foods which undergo sterilization treatment such as retorting or boiling, such as curry, stew, broth, sauce, and corn. The films are employed in the form of, for example, bag, casing, pouch, or capping material.